In the mevalonate biosynthetic pathway of most eukaryotes, farnesyl pyrophosphate (FPP) is incorporated into a wide variety of end-products, including dolichols, ubiquinone, hormones, haem A, sterols, and some isoprenylated proteins. Differential synthesis of these FPP-derived products is controlled through both regulated enzyme synthesis and differing affinities for FPP by the multiple enzymes that utilize this substrate. In mammalian cells, the enzymes of non-sterol polyisoprene synthesis possess high affinities for FPP but are synthesized at low constitutive levels. Conversely, squalene synthetase, the first committed enzyme of sterol biosynthesis, has a lower affinity for FPP but levels of this enzyme are regulated; a ten-fold depression in activity is seen in cells when they accumulate sufficient cholesterol. Together, these factors, in concert with regulation of mevalonate production by HMG CoA reductase (HMGR), ensure that adequate non-sterol products of FPP are made both in cells actively synthesizing cholesterol from FPP and in cells that receive most of their cholesterol exogenously, through uptake mediated by the low density lipoprotein (LDL) receptor. An understanding of how, at the molecular level, the cell achieves regulation of squalene synthetase activity is currently lacking.
Characterization of squalene synthetase has lagged behind that of many other enzymes of cholesterol metabolism due to intrinsic problems in working with this microsomal enzyme which, though present in many eukaryotes, is difficult to isolate and stabilize. The enzyme from the yeast Saccharomyces cerevisiae has been studied. It is typically solubilized with deoxycholate or nonionic detergents and after further purification, this enzyme is reported to have a monomeric molecular mass estimated at 55,000, 53,000, or 47,000. The solubilized enzyme usually retains the native enzyme's ability to condense two molecules of FPP into squalene in the presence of reduced pyridine nucleotides. Detailed characterization of the enzyme and its active site has been retarded by the poor yields of purified enzyme obtained.
It would be useful to have a source of large amounts of squalene synthetase to expedite its study and characterization.